Gracias DoHITB. Con eso me he conseguido lo que queria.
Aqui lo dejo por si le sirve a alguien.
Código PHP:
$array = array(3 => array(0 => '110', 1 => '451',2 => '655', 3 => '567'), 8 => array(0 => '102', 1 => '306 ',2 => '477', 3 => '164 '));
function media(&$keys1, &$keys2){
if( count($keys1)!=count($keys2) ) return array();
for ($i=0;$i<count($keys1);$i++)
{
$total = $keys1[$i] + $keys2[$i];
$key1[$i] = ($keys1[$i] / $total)*100;
$key2[$i] = ($keys2[$i] / $total )*100;
$res = array();
foreach($key1 as $key => $val )
{
$res[3][$key] = $val;
}
foreach($key2 as $key => $val )
{
$res[8][$key] = $val;
}
}
}
media($array[3], $array[8]);